/* Medium
You are given two non-empty linked lists representing two non-negative integers.
The digits are stored in reverse order, and each of their nodes contains a single digit.
Add the two numbers and return the carry as a linked list.

You may ascarrye the two numbers do not contain any leading zero, except the number 0 itself.

Example 1:
Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:
Input: l1 = [0], l2 = [0]
Output: [0]

Example 3:
Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

Constraints:
The number of nodes in each linked list is in the range [1, 100].
0 <= Node.val <= 9
It is guaranteed that the list represents a number that does not have leading zeros.

Relatives:
002. Add Two Numbers
445. Add Two Numbers II
415. Add Strings
043. Multiply Strings
066. Plus One
067. Add Binary
371. Sum of Two Integers
989. Add to Array-Form of Integer
1634. Add Two Polynomials Represented as Linked Lists */

struct ListNode {
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        int carry = 0;
        ListNode dummy(0);
        auto* tail = &dummy;

        while (l1 || l2) {
            if (l1) {
                carry += l1->val;
                l1 = l1->next;
            }
            if (l2) {
                carry += l2->val;
                l2 = l2->next;
            }
            tail = tail->next = new ListNode(carry % 10);
            carry /= 10;
        }

        if (carry) {
            tail = tail->next = new ListNode(carry);
        }

        return dummy.next;
    }
};